Wait on the patients but do them a bad turn? (5,3)

'wait on the patients' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'do them a bad turn?' is the wordplay.
I cannot quite understand how this works, but
'do' could be 'serve' (synonyms) and 'serve' is found in the answer.
'bad' could be 'ill' (associated in meaning) and 'ill' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).

'but' acts as a link.

Can you help me to learn more?